Dual CPU lets you have more cores of a particular family of processors. If you run a large amount of busy VMs concurrently then it might be handy.
However, this does not come for free. Compared to a single CPU with an equivalent number of threads, dual CPU has more complex memory access, and you don't want VMs and their memory to bounce between CPUs.
If you need it you'll know it. If you don't know if you need it then I would not recommend it.
Is a dual cpu mobo worthwhile for a homelab server
9 times out of 10, no.
In a world where a couple cheap NUC/Mini PCs loaded to the gills with 64 GB of RAM each is more than enough for a lab, dual processor builds are mostly for bragging rights and folks who don't have to contribute to the power bill 🥵
One thing to consider is that capability comes in many forms. Having multiple CPUs often means that you are transferring single-threaded performance to multi-threaded capacity. In commercial applications it's often a no-brainer, but in homelabs you often benefit of the raw performance quite a lot.
RAM and storage is where it's at.
I have 7 dual cpu servers so I might be a bit biased in this regard. But worthwhile is like entirely subjective. Robust is also a weird wordchoice since there's multiple conflicting interpretations on that.
For worthwhile... Well, it's as I said subjective, but cost efficiency is very rarely the driving factor for homelabs.
For robust, do you mean robust in the sense of more powerful? Then ofc a dual slot server will be more robust but then you again are back to worthwhile. If you mean robust in terms of stability. Then absolutely not. Multi socket servers are much less stable than single core. Not unstable by any stretch, but not AS stable. Every additional component you add will always add complexity, and most importantly, additional points of possible failures. While at the same time, the system can't survive if one CPU dies, hence stability of the system is lower the more CPU sockets you have. That's why dual and quad are so popular even if 8 slot and more actually existing and is denser which is important in datacenters. But after quad slot, you start getting actual issues of system stability that it's usually better to sacrifice some density and go for more servers instead and blade centers are usually not THAT much lower density.